This invention relates to hydraulic pumping systems for pumping well fluids, and more particularly to reducing the power consumption of such a hydraulic pumping unit.
Hydraulically actuated downhole pumps have been used rather than beam-pumping units in many locations. Hydraulic pumping units are especially attractive in the deeper and higher producing wells.
A hydraulic pumping unit uses a prime mover to drive an above-ground pump (typically, a triplex pump) and this pump supplies a flow of pressurized fluid, at least some of which pressurized fluid is used as power fluid for a downhole hydraulically actuated pump. The downhole pump returns to the surface fluid which is at least some of the power fluid together with produced well fluids. At least some of this return fluid is introduced into a cyclone separator which conditions some of this fluid and makes the conditioned fluid available to the above-ground pump for use as power fluid. The remainder of the cyclone flow (the portion containing the separated solids) is sent to a flowline, where this cyclone underflow and any return fluid which was not sent to the cyclone are combined to become the production from the well.
The speed of the downhole pump has been controlled by varying the amount of power fluid which is supplied to the downhole pump. As the triplex pump is generally driven directly by a conventional AC motor, the speed of such pump and therefore the flow therethrough is fairly constant. The pump speed in such systems will, of course, vary slightly with pump head, but is not normally varied to change the flow through the pump. When special arrangements such as transmissions have allowed the flow rate through the triplex to be changed (an approximately 2:1 ratio has been found to be desirable), such a variation requires either shutting down the well and modifying the cyclone or to very poor cleaning in the cyclone leading to excessive and expensive wear of the downhole pump (cyclones only clean properly if the flow is within about 10% of the rated value). As a result, conventional systems generally do not control the speed of the triplex pump, but instead bypass a portion of the flow through a throttling valve (the pressure is reduced from several thousand psi to approximately 100 psi by the throttling valve). The bypass fluid is then combined with the return fluid from the well.
Hydraulic pumping systems are described in, for example, U.S. Pat. Nos. 2,046,769, 2,119,737, and 2,593,729, issued to Coberly; U.S. Pat. Nos. 3,759,324 and 3,802,501, issued to Mecusker; and U.S. Pat. Nos. 3,709,292 and 3,782,463, issued to Palmour.
Although systems such as described in the aforementioned Pat. No. 3,802,501 have recirculated some fluid to improve cleaning of the fluid, apparently all of these systems have been used with triplex pumps driven at an essentially constant speed (i.e., by an AC motor).